2nd Level Buffer Cache

Hi,

I have implemented initial concept of 2nd level cache. Idea is to keep some 
segments of shared memory for special buffers (e.g. indices) to prevent 
overwrite those by other operations. I added those functionality to nbtree 
index scan.

I tested this with doing index scan, seq read, drop system buffers, do index 
scan and in few places I saw performance improvements, but actually, I'm not 
sure if this was just "random" or intended improvement.

There is few places to optimize code as well, and patch need many work, but 
may you see it and give opinions?

Regards,
Radek